1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a sheet.
2. Description of the Related Art
Heretofore, there has been known an image forming apparatus in which a toner image formed on an image bearing member, such as a photosensitive member or an intermediate transfer member, is transferred onto a sheet by a transfer unit and the toner image transferred onto the sheet is heated and fixed by a fixing unit.
The fixing unit applies heat which is suited for melting and fixing the toner onto the sheet in the fixing unit. The quality of heat varies according to the kind of the sheet. According, an apparatus changes the conveyance speed of a sheet in a fixing unit according to the kind of a sheet, which changes the quantity of heat applied to the sheet according to the kind of the sheet.
Japanese Patent Application Laid-Open No. 2006-251441 discusses an image forming apparatus which makes faster the conveyance speed of a sheet in a fixing unit than in a transfer unit in a case where the sheet is plain paper (whose thickness is normal). The image forming apparatus arranges two conveyance belts side by side in the conveyance direction between the transfer and fixing units. The two conveyance belts convey the sheet while sucking the sheet to convey the sheet holding the toner yet to be fixed to the fixing unit. Each length of the two conveyance belts in the direction in which the sheet is conveyed is longer than the maximum length of a usable sheet. In a configuration described in Japanese Patent Application Laid-Open No. 2006-251441, the sheet is conveyed as described below.
The sheet sent out from the transfer unit is received by an upstream conveyance belt driven at a conveyance speed in the transfer unit and the upstream conveyance belt conveys the sheet to a downstream conveyance belt similarly driven at the conveyance speed in the transfer unit. Before the leading edge of the sheet reaches the fixing unit and when the trailing edge of the sheet reaches the downstream conveyance belt, the conveyance speed of the downstream conveyance belt is changed to a fixing speed. The downstream conveyance belt driven at the fixing speed conveys the sheet to the fixing unit.
More specifically, the length of the downstream conveyance belt in the sheet conveyance direction is longer than the maximum length of the usable sheet, which sometimes carries and conveys the sheet only by the downstream conveyance belt, and, at this point, the conveyance speed of the downstream conveyance belt is changed to the fixing speed. In the above image forming apparatus, however, when the trailing edge of the sheet to be conveyed reaches the downstream conveyance belt, the conveyance speed of the downstream conveyance belt is changed to a high fixing speed. In other words, the sheet is conveyed at a low transfer speed until the trailing edge of the sheet to be conveyed moves away from the upstream conveyance belt. This delays the timing at which the conveyance speed of the sheet is changed to the high fixing speed. The timing at which the speed of the sheet advancing from the transfer unit is changed to the high speed is slow, taking the sheet advancing from the transfer unit longtime to reach the fixing unit, which lowers productivity.